miRNAs are small RNA molecules with a length of 20-22 nt (nucleotide), present in eukaryotes and encoded by the genomes of the eukaryotes. MiRNAs recognize target genes mainly by complementarily pairing with the RNA of target genes and then inhibit the expression of the target genes through miRNA-RISC (RNA induced silence complex) (Jones-Rhoades M W, Bartel D P, and Bartel B. MicroRNAs and their regulatory roles in plants. Annual Review of Plant Biology, 2006, 57: 19-53). Each miRNA gene produces at least three small RNA species, including a long primary miRNA transcript (pri-miRNA), an intermediate ˜60 nt precursor miRNA (pre-miRNA), and a ˜21 nt mature miRNA, through sequential endonucleolytic maturation steps (Kim VN MicroRNA biogenesis: coordinated cropping and dicing. Nat Rev Mol Cell Biol 2005, 6: 376-385). The transcripts of miRNA genes (pre-miRNAs) are structurally consistent with common mRNAs, having such structures as 5′-CAP and 3′ poly(A). Therefore pre-miRNAs can also be cloned from conventional cDNA library. Common miRNAs have no protein-encoding region in their transcripts. While pre-miRNA molecules may have several very small ORFs, no pre-miRNA molecules from which a protein can be translated have been found. Pre-miRNAs from which miRNAs are formed are located in the transcripts of miRNA genes, and they have a length of 60 nt to 200 nt, or more than 200 nt for some of them. Pre-miRNAs can form a stable foldback secondary structure that is recognized by an enzyme necessary for miRNA maturation. MiRNAs play very important regulatory roles during development and growth of a plant, involving in various aspects of plant development, growth, and biological and non-biological stresses. The target genes of many miRNAs belong to transcription factor family. The same miRNA may often inhibit the functions of a variety of target genes, while regulating various interconnected processes during plant development and growth. For example, overexpression of miR156 increases the number of leaves of Arabidopsis thaliana more than 100 times and plant dry weight 5 times, and delays flowering time (Wu G and Poethig R S. Temporal regulation of shoot development in Arabidopsis thaliana by miR156 and its target SPL3. Development, 2006, 133: 3539-3547). In corn, miR172 regulates the sex differentiation of flower organ in addition to flowering time (Chuck G, Meeley R, Irish E, Sakai H, and Hake S. The maize tasselseed4 microRNA controls sex determination and meristem cell fate by targeting Tasselseed6/indeterminate spikelet1. Nat Genet, 2007, 39: 1517-1521). MiR159 has an important regulatory role in ABA response, seed germination, flower development and leaf shape, etc. Overexpresion of miR159 in plants will result in the plant's reduced sensitity to abscisic acid (ABA) during the germination period, delayed flowering time and decreased anther fertility (Reyes J L and Chua N H. ABA induction of miR159 controls transcript levels of two MYB factors during Arabidopsis seed germination. Plant J, 2007, 49: 592-606).
Rice is the most important cereal crop. Altering the morphology, fertility and flowering time of rice plants is of great significance for increasing rice yield. MiRNAs are a key regulatory factor during plant development and growth. They generally can alter many key traits of plants, especially for those highly conserved miRNAs. The development of plant root system is critical for the plant to absorb nutrients and water. A plant with a large root system will have increased absorption of nutrients and water and enhanced resistance to stresses. On the other hand, controlling plant fertility will be of great significance and usefulness for breeding hybrid crop varieties by virtue of heterosis.